As a device which drives a rotary machine, such as a compressor, there are an electric device which generates a rotation driving force and a transmission device which changes the speed of the rotation driving force generated by the electric device and transmits the rotation driving force to the rotary machine.
Patent Document 1 describes that a constant-speed electric motor and a variable-speed electric motor for speed change are used as the electric device and a planetary gear transmission device is used as the transmission device in order to accurately control a gear ratio. In the device, by changing the rotational speed of the variable-speed electric motor, it is possible to change the rotational speed of an output shaft of the planetary gear transmission device connected to the rotary machine.